While specific formula is not given, theymos wrote that the initial sMerit is not solely determined by rank, but determined by rank and activity points obtained in the last year.
The spendable amount was calculated based on your current rank and the number of activity points you earned in the last year. A Legendary member who hasn't posted in the last year would still be Legendary, but would not have any spendable merit.
I know a Sr. Member who had 26 initial sMerit when the merit system was introduced, but it fluctuates for each user.